In the very body of the rocks also is hidden a secret enemy; the salt left when they were made. And more salt is constantly being forced into the surface pores as the waves strike. This salt helps to dissolve and weaken the rock under the chemical action of the air, and the rains and the mechanical expansion and contraction of the surface with changes of temperature. PLANING MILLS OF THE WINTER SEA All the Great Powers of nature, "on land, on sea, and in the air," seem to be in open conspiracy against our peace. The evidence seems especially plain in late fall and winter, when the sea, contrary to the usual practice in war, carries on its most vigorous campaigns. Then come the winds for the great drives; then come the frosts that change the water wedges into expanding blocks of ice that, almost with the force of exploding shells, tear the walls apart. In winter are formed the great ice-fields that help in two ingenious ways to further the destructive action of the storm waves. In bays and smaller recesses in rocky shores, the ice has embedded in it fragments of stone which the sea has battered down. The constant plunge of the waves breaks up these ice-fields into sections which, with the embedded stones, become rude planing mills. Where a headland is sloping, these planers, driven back and forth by the waves, chisel the rock away as a planer chisels down a piece of steel upon which it has been set to work. HOW STONES ARE CARRIED OUT TO SEA A no less curious feature of sea engineering is the use of ice-fields as "conveyors." During the spring, summer, and autumn the masses of stone which the sea brings down from the cliffs on its occasional busy days--that is to say on days when the winds are high--pile up and so form a kind of bulwark against further attacks. But when in winter these stones become embedded as above described, strong offshore winds carry the ice-fields, stones and all, out to sea. Then, on shore, wind and wave take up their work again unchecked. All along the rocky shores of the Atlantic, as far south as New York State, beyond which no rock walls come down to the shore, all these interesting things may be seen by the traveller.